Nan Dieter Conklin: A Life in Science

UC Berkeley: Galactic HI

The major programs of the radio astronomy laboratory at Berkeley during these years were large-scale surveys in the 21-cm hydrogen-line radiation. The object was to study the overall structure and dynamics of our galaxy. For the purpose, the relatively small telescope was not a disadvantage, and the very sensitive receivers developed at this wavelength were a great advantage. The first survey (by Weaver and Williams) covers a strip centered on the galactic plane, extending to latitudes ten degrees above and below the plane. The second (by Heiles and Habing) includes all latitudes outside this range that are visible from Hat Creek. These form a complete sky survey, limited in that they cover gas only at relatively low velocities with respect to the sun. My work extended this coverage to high-velocity gas. Altogether the program required a major commitment of time and resources. For example, my survey required about 2500 hours of observing, and that was only the beginning. Recording, organizing, and interpreting the mass of data required far longer.

The observations were made during several periods from May 1969 to August 1970. At each of the about 4750 positions on the sky the intensity of the hydrogen-line emission was recorded in 100 separate channels of the receiver, each representing a range of 2 km/sec in radial velocity. This mass of data was handled in a way that now seems "prehistoric". It was recorded at the Observatory on punched cards that were sent to the lab in Berkeley where they were fed into the main-frame computer (that took up a large part of the basement of the mathematics building.) The programs designed to calibrate and synthesize the data were also fed in on punched cards. (This entailed a lot of carrying boxes of cards.) Fortunately for me, these programs had to a great extent been worked out for the other hydrogen- line surveys at the lab. The final presentation of the data is in the form of contour maps of the sky showing the intensity of the emission as a function of radial velocity. The result was about as awkward as my description, and is done much more elegantly these days.

The observations are by their nature uncertain. My particular interest was in determining at each point in the sky the highest velocity at which emission was detectable. This means, of course, that the results depended on detection of the weakest signals -- those just above the zero level of the equipment. Unfortunately, for a variety of reasons the observations have not been checked by my colleagues.

The model that I proposed to explain my observations of high-velocity neutral hydrogen is a simplified one but also one that seems to work. The idea is that the high-velocity gas lies at the periphery of our galaxy, outlines the outermost spiral arms, and extends to considerable distances above and below the plane of the galaxy. This gas has a rotational velocity appropriate to its distance from the galactic center, and in addition has a component of velocity directed toward the center with a magnitude of 125 km/sec. The most obvious characteristic of the observations that it explains is the overwhelming preponderance of negative velocities (indicating material approaching the sun). Because we lie within the shell all the material in it would appear to be approaching us. One possibility is that the high-velocity gas is part of the normal galactic material that has been set in motion by the inflow of extragalactic gas. Its inward motion is a remnant of the conditions that led to the formation of the galaxy (and all galaxies).

With the much greater sensitivity of today's instruments one might imagine testing the existence of the high-velocity shell by observing other galaxies. However, nature conspires against such a test. If the galaxy is observed face-on (that is, perpendicular to the line of sight), there should be virtually no evidence of the gas (assumed to lie close to the plane). And if the galaxy is observed edge-on, any high-velocity gas would be hopelessly confused with the other galactic motions.